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Texas Fire Weather Operating Plan

Weather Parameters used in NWS products

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The following sections describe the various forecast parameters that are provided in the fire weather forecast products issued by NWS offices having CWFA responsibilities in Texas. Which parameters are used will be determined by the individual NWS offices as they tailor their services to fit the needs of the user agencies. NWS offices are obligated to provide user agencies with units of measure and/or a legend to explain ambiguous weather parameters.

 Weather Trends and Changes   
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The NWS and the user agencies acknowledges that forecasting drastic weather changes such as fronts, drylines and other wind shifts, and timing of precipitation and thunderstorms is the most critical service that the NWS can provide. These elements all can be described qualitatively in a brief narrative portion of fire weather forecasts called the weather synopsis or map discussion. Forecasters are strongly encouraged to highlight any weather changes which could pose a threat to land management or fire control efforts in a headline statement preceding the narrative discussion. Synoptic discussions should focus mainly on changes in weather conditions that would impact land management activities and planning.

Wind, humidity and boundary interaction
A change in wind direction is perhaps the most critical influence on fire control efforts. A wind shift during a Red Flag event could result in loss of life or cost land management agencies and property owners millions of dollars. Subtle changes in wind direction can often lead to changes in humidity, which in turn could have a significant effect on the spread of wildfires. Forecasters are encouraged to forecast the timing of frontal boundaries and wind shifts in the synopsis if they have a reasonable level of confidence. Information on wind, humidity and boundary interaction may also be described in a tabular format, but should be complimented by a narrative discussion if the forecaster expects a complex interaction of boundaries.
Precipitation and thunderstorms
While being expected to provide precipitation chances based on probability of precipitation (as is provided in the zone forecast product), forecasters are also encouraged to evaluate the likelihood of a wetting rain in the synopsis portion of the forecast. A wetting rain is a widespread rain that over an extended period of time significantly reduces fire danger. NWS fire weather program leaders should ensure their user agencies understand how a wetting rain is discussed in the forecast. Note: NWS offices needing to decide on specific values to assign to wetting rains for their CWFA should consult with nearby fire behavior specialists. When thunderstorms are expected, forecasters are encouraged to describe the type of weather that can be expected in or near thunderstorms, i.e. heavy rains, outflow boundaries, dry lightning, etc., in the narrative section of the forecast.
Sky condition
Sky condition trends discussed qualitatively in the synopsis of a forecast can also give the user agencies a better understanding of how other weather variables will be affected. Sky condition trends may also be described in a tabular assessment expressed as a percentage of sunlight penetrating the clouds in a day or as minutes of sunlight.
Other weather phenomena
Smoke, fog, and dust expected to create significant problems for wildfire control efforts should be included in the synopsis of the forecast. Severe weather, winter weather, and flash flood events are unlikely to occur during extreme wildfire events but could still be of interest to the user agencies for wildland planning efforts.
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Wind speed and direction is generally indicated for the most hazardous part of the day or at other times specified in the forecast. The NOAA/NWS Operational Manual Chapter D-6 states "Users of fire weather forecasts should be made aware of the level for which the wind is forecast, i.e., eye-level, 20 feet, free-air, etc." Maximum gusts, erratic winds, and wind shifts should be mentioned when expected.

20-foot winds
Winds at 20 feet above the ground or above the average height of vegetation are the most common winds used in the routine fire weather forecast. Since most surface stations used for NWS forecasts measure the wind at 33 feet, a reduction factor is needed to arrive at the 20-foot wind. FTS/RAWS sites, which measure 20 foot wind speed and direction, can be used to compare the 33 foot winds, but are available for only a few NWS offices with responsibilities in Texas.
Eye-level winds
Eye-level (or 6-foot) winds are often used for spot forecasts to compliment preliminary reports taken at the burn site. These wind forecasts may also be estimated using a reduction factor to the available surface wind data.
Transport winds / ventilation index
Average winds in the mixing layer and the depth of the mixing layer are parameters that are helpful for land management agencies to evaluate the potential for very large fires and also for smoke dispersal. Data computed from morning atmospheric soundings and model forecast soundings are used to provide ventilation values for periods of maximum heating. The following are terms and definitions necessary to understanding ventilation data and values...
  • Mixing height or mixing depth
    The height to which relatively vigorous mixing occurs due to heating. Units are in feet above ground level (AGL), with ground level being the elevation above mean sea level (MSL) of the upper-air site. It is important that wildland fire managers note the difference in elevation between the burn site and the referenced upper-air site, and modify the provided mixing depths accordingly.
  • Transport winds
    A measure of the average rate of the horizontal transport of air within the mixing layer. Units can be expressed in knots (1 knot = 1.15 mph) or mph. An average wind direction (the direction from which the wind is blowing) is provided. If winds are light and variable, then it may be best to consider local drainage effects when in critical situations.
  • Ventilation
    The product of the mixing height and the transport wind speeds. It is a measure of the volume rate of horizontal transport of air within the mixing layer per unit distance normal to the winds. Units are in knot-feet. As a guide, the following categories have been established to describe the ventilation...

    Excellent 150,000 kt-ft or greater
    Very Good 100,000-149,999 kt-ft
    Good 60,000-99,999 kt-ft
    Fair 40,000-59,999 kt-ft
    Poor less than 40,000 kt-ft

    When ventilation values are less than 40,000 kt-ft along with transport winds of less than 7.0 knots, dispersion of any pollutants released into the atmosphere will be severely limited.
 Relative Humidity   
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Relative humidity is the ratio (expressed in %) of the amount of water vapor actually in the air compared to the amount the air is capable of holding at its temperature and pressure. Since relative humidity values are also critical to fire management activities, they should always be included in routine and spot forecasts. Relative humidity values can vary greatly over a small area due to variations in topography, vegetation and location with respect to bodies of water. Therefore, a range of values will often be used in routine fire weather forecasts, but forecasters should make an attempt to narrow this range when making Spot Forecasts.

 Lightning Activity Level   
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A Guide for Fire Weather Observers

LAL Cloud & Storm Development Individual storm cell cloud-to-ground (cg) lightning discharges
cg / 5 min
cg / 15 min
cg / min
1 No thunderstorms n/a n/a n/a n/a
2 Cumulus clouds are common but only a few reach the towering stage. A single thunderstorm must be confirmed in the rating area. The clouds mostly produce virga but light rain will occasionally reach ground. Lightning is very infrequent. <15% 1-5 1-8 <1
3 Cumulus clouds are common. Swelling and towering cumulus cover less than 2/10 of the sky. Thunderstorms are few, but 2 to 3 occur within the observation area. Light to moderate rain will reach the ground, and lightning is infrequent. 15% to 24% 6-10 9-15 1-2
4 Swelling cumulus and towering cumulus cover 2-3/10 of the sky. Thunderstorms are scattered but more than three must occur within the observation area. Moderate rain is commonly produced, and lightning is frequent. 25% to 50% 11-15 16-25 2-3
5 Towering cumulus and thunderstorms are numerous. They cover more than 3/10 and occasionally obscure the sky. Rain is moderate to heavy, and lightning is frequent and intense. >50% >15 >25 >3
6 Dry lightning outbreak. (LAL of 3 or greater with majority of storms producing little or no rainfall.) n/a n/a n/a n/a

 Haines Index   
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The Haines Index (HI) is a numerical value that indicates the potential for large wildfires to experience extreme fire behavior (i.e. crowning, spotting, and rapid rates of spread). The HI combines both the instability and dryness of the air by examining the lapse rate between two pressure levels in the atmosphere and the dryness at one of the pressure levels. There are three different methods of computing HI depending upon whether the area elevation is considered low, medium or high. Each NWS office determines the elevation which is most suitable for their area of responsibility. For each elevation, Haines Index classifications are assigned to values 2 through 6 as shown below...

Haines Index Potential for Large Fire Growth
2 or 3 Very Low
4 Low
5 Moderate
6 High

The HI numbers are computed for each elevation using the following parameters...

Low Elevation HI = 950-850 MB TEMP
A=1 when 3°C or less
A=2 when 4-7°C
A=3 when 8°C or more
B=1 when 5°C or less
B=2 when 6-9°C
B=3 when 10°C or more
Mid Elevation HI = 850-700 MB TEMP
A=1 when 5°C or less
A=2 when 6-10°C or less
A=3 when 11°C or more
B=1 when 5°C or less
B=2 when 6-12°C or less
B=3 when 13°C or more
High Elevation HI = 700-500 MB TEMP
A=1 when 17°C or less
A=2 when 18-21°C
A=3 when 22°C or more
B=1 when 14°C or less
B=2 when 15-20°C
B=3 when 21°C or more

 Inversion Burn-off   
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Information on inversion burn-off time and/or temperature is an optional forecast parameter that many user agencies may request. Since eroding inversions are often highly variable over a small area, forecast inversion burn-off times and temperatures will be most accurate and useful when used in site-specific weather forecasts.

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